1. Field of the Invention
Embodiments disclosed herein relate generally to the recovery of gaseous hydrocarbons. In particular, embodiments disclosed herein relate to the recovery of gaseous hydrocarbons from hydrate-capped gas reservoirs.
2. Background Art
Production of gas from subterranean oil and gas reservoirs is a well-established practice. Natural gas (primarily methane) production has for the most part been achieved through drilling wells into deep reservoirs where natural gas, frequently in association with condensate, crude oil, and water, may be trapped under a layer of cap rock. The well is lined with a casing that is cemented to the surrounding formation to provide a stable wellbore. The casing is then perforated at the reservoir level to allow gas and reservoir fluids to flow into the casing and then to the surface through tubing inside the casing.
After entering the casing via the perforations, the gas enters the tubing string(s) where it flows to the surface, through valves, and to a pipeline. The cased well method facilitates control of the flow of gas from a high-pressure reservoir and is well suited for production from porous rock or sand formation material. If the reservoir has sufficient integrity, the producing formation may not need to be stabilized with casing, and production may be initiated through various types of open-hole completions.
Gas hydrates are clathrates (inclusion compounds) in which small hydrocarbon molecules (as well as CO2, H2S, and N2) are trapped in a lattice consisting of water molecules. Frozen water particles form an expanded crystalline structure that traps methane, or other particles. Gas hydrates form exothermically as a consequence of the tendency of water to reorient in the presence of a non-polar solute (typically light hydrocarbon gases such as methane) to stabilize the lattice through, typically, van der Waals interactions while maintaining the hydrogen bonding between the water molecules. Tetra-hydrofuran, p-dioxane, CO2, and H2S, to name a few other compounds, in addition to the low-molecular-weight hydrocarbons are capable of occupying the interior positions in a clathrate lattice of water molecules and stabilizing the overall structure so that it does not decompose until a relatively substantial increase in temperature or decrease in pressure occurs or both occur.
Methane hydrates form at elevated pressures and at temperatures much higher than the freezing point of water. They can be stable over broad ranges of pressure and temperature. Methane hydrates are stable at combinations of temperature and pressure found in onshore arctic regions and beneath the sea floor in water depths greater than approximately 1,500 feet (500 meters). Changes in either the temperature or the pressure can cause methane hydrates to melt and release natural gas. Methane gas may also be trapped below the hydrate layer, much as it is trapped below cap rock layers in deep underground reservoirs.
Gas hydrate encountered during drilling are a potential source of hydrocarbons for exploitation and production. Thus, the development of viable methods for the commercial production of natural gas from naturally occurring deposits of methane hydrates has been the subject of extensive research. Such techniques may include depressurization, thermal injection, and inhibitor injection. However, dissociation of hydrates can cause instability in the neighboring rock, which often is a poorly consolidated mixture of sediment and hydrate or of sediment and ice referred to as “permafrost.”
In addition to the methane trapped in a gas hydrate layer, there may often be a petroliferous reservoir located beneath the hydrate layer. When producing hydrocarbons from the reservoir located below the hydrate deposit, particular attention must be paid to unintended but unavoidable reduction of pressures which may cause the hydrates to decompose, weakening the trapping layer, and creating a risk of destabilizing the wellbore, as well as the reservoir.
Accordingly, there exists a continuing need for development in production techniques for recovering hydrocarbons from hydrate-capped reservoirs.